linux-kernel - Re: [PATCH 1/2] livepatch: Implement separate coming and going module notifiers

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Date:	Fri, 29 Jan 2016 14:15:34 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Miroslav Benes <mbenes@...e.cz>, Jessica Yu <jeyu@...hat.com>,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>,
	Ingo Molnar <mingo@...hat.com>, live-patching@...r.kernel.org,
	linux-kernel@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 1/2] livepatch: Implement separate coming and going
 module notifiers

On Fri, Jan 29, 2016 at 03:08:23PM -0500, Steven Rostedt wrote:
> On Fri, 29 Jan 2016 13:47:15 -0600
> Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
> 
> > > Although, I have to admit, if live kernel patching is configured in,
> > > it's not always needed to be called here, does it? With ftrace, the
> > > call has to be done when ftrace is configured in regardless if tracing
> > > is used or not.  
> > 
> > For live patching it actually does need to be called for every module.
> > We need to check if any previously loaded patches have any modifications
> > which affect the module.
> > 
> 
> But only if you are using the live kernel patching facility. My point
> is, if I never use live kernel patching (which 99.9% of Linux users do
> no use), then the call will basically be a nop.
> 
> With ftrace, that's not the story. Even if you don't ever use the
> facility (like 99.8% of Linux users do not use ;-) the function is
> still not a nop. There's three calls needed for each module.
> 
> 1) convert all the calls to mcount/fentry into a nop, and save
> those locations in a table. They are all marked as disabled (not to be
> used)
> 
> 2) After module setup (where the notifiers are called), the locations
> need to be enabled, otherwise ftrace would never work for that module.
> 
> 3) On module exit, the locations must be removed, otherwise ftrace may
> still try to write to non-existing code which could brick specific
> network cards.
> 
> These are done if ftrace is configured regardless if it is every
> actually used by a user.
> 
> Thus, my question is, what does live kernel patching need to do if I
> never add a patch?

Right, as you say it's basically a nop 99.99% of the time.  But we still
need to do the "are any patches loaded" check, so we still need the call
into a livepatch function to do that.

-- 
Josh